Americans spend more than $30 billion a year on dietary supplements — vitamins, minerals and herbal products, among others — many of which are unnecessary or of doubtful benefit to those taking them. That comes to about $100 a year for every man, woman and child for substances that are often of questionable value.

The passage of the Dietary Supplement Health and Education Act of 1994 opened the floodgates to an industry that can bring these products to market without submitting any evidence to the Food and Drug Administration that they are safe and effective in people. The law allows the products to be promoted as “supporting” the health of various parts of the body if no claim is made that they can prevent, treat or cure any ailment. The wording appears not to stop many people from assuming that “support” translates to a proven benefit.

After 1994, sales of a very wide range of supplements skyrocketed, and because the law allowed it, many continued to be sold even after high-quality research showed they were no better than a placebo at supporting health. The government can halt sales of an individual product only after it is on the market and shown to be mislabeled or dangerous.

The latest study, published in October in JAMA, found that overall use of dietary supplements by adults in this country has remained stable from 1999 through 2012, although some supplements have fallen out of favor while the use of others has increased.

The study, directed by Elizabeth D. Kantor, an epidemiologist at Memorial Sloan Kettering Cancer Center in New York, revealed that 52 percent of adults used one or more supplements in 2012. If anything was surprising about the findings, it was that the number of supplement users was not even higher given the products’ robust promotion in paid advertisements and testimonials on the internet.

In an accompanying editorial titled “The Supplement Paradox: Negligible Benefits, Robust Consumption” accompanying the new report, Dr. Pieter A. Cohen, of Cambridge Health Alliance and Somerville Hospital Primary Care in Massachusetts, pointed out that “supplements are essential to treat vitamin and mineral deficiencies” and that certain combinations of nutrients can help some medical conditions, like age-related macular degeneration. He added, however, “for the majority of adults, supplements likely provide little, if any, benefit.”

Among the changes found in the new study: multivitamin/mineral use declined to 31 percent from 37 percent, “and the rates of vitamin C, vitamin E and selenium use decreased, perhaps in response to research findings showing no benefit,” Cohen wrote. Sometimes people do act sensibly when faced with solid evidence.

However, he added, “other products continued to be used at the same rate despite major studies demonstrating no benefit over placebo.” Thus, the use of glucosamine-chondroitin to relieve arthritic pain remained unaffected by the negative results in 2006 of the Glucosamine/Chondroitin Arthritis Intervention Trial and several follow-up analyses.

Others have studied who uses dietary supplements and why. Again using data from the National Health and Nutrition Examination Survey of 2007-10 that included 11,956 adults, Regan L. Bailey of the National Institutes of Health Office of Dietary Supplements and co-authors reported in 2013 these reasons given by the survey participants: 45 percent said they took them to “improve” and 33 percent to “maintain” overall health. Thirty-six percent of women took calcium for bone health and 18 percent of men took supplements for heart health or to lower cholesterol. Only 23 percent used supplements because a health care provider suggested they do so.

Perhaps most enlightening were the data on the characteristics of supplement users. In all probability, they were among the healthiest members of the population. They were more likely than nonusers to report being in very good or excellent health, to use alcohol moderately, to refrain from cigarette smoking, to exercise frequently and to have health insurance. Other studies have shown that supplement use is also more frequent among those who are older, who weigh less and have higher levels of education and socioeconomic status.

This means that in trying to determine possible health benefits of a supplement, researchers must control for all such characteristics in order to isolate the contribution of the supplement. Just looking at a large group of people, even following them for decades and finding that supplement users were healthier or lived longer, proves nothing if other influences on health and longevity are not taken into account.

Faced with equivocal or negative findings of health benefits from supplements, in 2013 the U.S. Preventive Services Task Force, an independent group of physicians who base their advice on solid evidence, opted not to recommend the regular use of any multivitamins to prevent cardiovascular disease or cancer in people who were not nutrient deficient.

All of which makes one wonder why people, myself included, opt to take one or more dietary supplements. Those who take a daily multivitamin/mineral supplement typically cite “nutritional insurance” as their rationale. Knowing that they often eat erratically or fail to consume recommended amounts of nutrient-rich vegetables and fruits, a supplement containing a broad range of vitamins and minerals seems the easiest and cheapest way to fill in any gaps.

But nutrition specialists point out that no pill can supply all the nutrients found in wholesome foods. For example, a multivitamin/mineral supplement contains none of the fiber in fruits and vegetables, and to provide the amount of daily calcium recommended (1,000 milligrams for adults, rising to 1,200 for women older than 50 and men older than 70), the combination pill would be too big for most people to swallow.

Some supplement users distrust evidence suggesting they have no benefit, which is why I still take glucosamine/chondroitin despite the results of the best study to date that found it offered no relief from knee arthritis. My arthritis has progressed minimally in the decades I’ve been on it, and having experienced no side effects, I’m unwilling to argue with apparent success.

Still, a cautionary approach to supplements is wise. Some can be harmful or interfere with prescribed medication. Tell your doctor what you take and be sure the information is recorded in your chart.